The point here is that "false" is not a reserved word (or a built-in function) in Perl, so it has no special meaning. In your snippet, it's treated as a "bareword", which you can look up under "B" in the perlglossary:

A word sufficiently ambiguous to be deemed illegal under use strict 'subs'. In the absence of that stricture, a bareword is treated as if quotes were around it.

In other words:

perl -le '$x = false; print length($x); print "x is ".join("-",split//
+,$x)'
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(that will print out two lines: "5" and "x is f-a-l-s-e")